Thermal transfer printing is widely used in special applications such as in the printing of machine readable bar codes, either on labels or directly on articles to be encoded. The thermal transfer process employed by these printing methods provides great flexibility in generating images allowing for broad variations in the style, size and color of the printed images, typically from a single machine with a single thermal print head.
Representative documentation in the area of multi-layer thermal transfer printing includes the following patents:
U.S. Pat. No. 4,315,643, issued to Y. Tokunaga et al. on Feb. 16, 1982, discloses a thermal transfer element comprising a foundation, a color developing layer and a hot melt ink layer. The ink layer includes heat conductive material and a solid wax as a binder material.
U.S. Pat. No. 4,403,224, issued to R. C. Winowski on Sep. 6, 1983, discloses a surface recording layer comprising a resin binder, a pigment dispersed in the binder, and a smudge inhibitor incorporated into and dispersed throughout the surface recording layer, or applied to the surface recording layer as a separate coating.
U.S. Pat. No. 4,523,207, issued to M. W. Lewis et al. on Jun. 11, 1985, discloses a multiple copy thermal record sheet which uses crystal violet lactone and a phenolic resin.
U.S. Pat. No. 4,698,268, issued to S. Ueyama on Oct. 6, 1987, discloses a heat resistant substrate and a heat-sensitive transferring ink layer. An overcoat layer may be formed on the ink layer.
U.S. Pat. No. 4,707,395, issued to S. Ueyama et al. on Nov. 17, 1987, discloses a substrate, a heat-sensitive releasing layer, a coloring agent layer, and a heat-sensitive cohesive layer.
U.S. Pat. No. 4,778,729, issued to A. Mizobuchi on Oct. 18, 1988, discloses a heat transfer sheet comprising a hot melt ink layer on one surface of a film and a filing layer laminated on the ink layer.
U.S. Pat. No. 4,869,941, issued to Ohki on Sep. 26, 1989, discloses an imaged substrate with a protective layer laminated on the imaged surface.
U.S. Pat. No. 4,894,283, issued to Wehr on Jan. 16, 1990, discloses a reusable thermal transfer ribbon with a functional layer and a binding layer containing 100% ethylene vinyl acetate copolymer.
U.S. Pat. No. 4,923,749, issued to Talvalkar on May 8, 1990, discloses a thermal transfer ribbon which comprises two layers, a thermosensitive layer and a protective layer, both of which are water based.
U.S. Pat. No. 4,975,332, issued to Shini et al. on Dec. 4, 1990, discloses a recording medium for transfer printing comprising a base film, an adhesiveness improving layer, an electrically resistant layer and a heat sensitive transfer ink layer.
U.S. Pat. No. 4,988,563, issued to Wehr on Jan. 29, 1991, discloses a thermal transfer ribbon having a thermal sensitive coating and a protective coating. The protective coating is a wax-copolymer mixture which reduces ribbon offset.
U.S. Pat. Nos. 5,128,308 and 5,248,652, issued to Talvalkar, each disclose a thermal transfer ribbon having a reactive dye which generates color when exposed to heat from a thermal transfer printer.
U.S. Pat. No. 5,240,781, issued to Obata et al. on Aug. 31, 1993, discloses an ink ribbon for thermal transfer printers having an ink layer with viscosity, softening and solidifying characteristics said to provide clear images on rough paper even with high speed printers.
U.S. Pat. No.5,567,506, issued to Sogabe on Oct. 22, 1996, discloses a thermal ribbon having a release layer with a melt viscosity below 1000 cps and colored ink layer with a high melt viscosity.
The coating formulations which provide thermal transfer ribbons vary widely. Representative documentation in this area includes the following patents:
U.S. Pat. No. 3,663,278, issued to J. H. Blose et al. on May 16, 1972, which discloses a thermal transfer medium having a coating composition of cellulosic polymer, thermoplastic resin, plasticizer and a "sensible" material such as a dye or pigment.
U.S. Pat. No. 4,628,000, issued to S. G. Talvalkar et al. on Dec. 9, 1986, discloses a thermal transfer formulation that includes an adhesive-plasticizer or sucrose benzoate transfer agent and a coloring material or pigment.
U.S. Pat. No. 4,687,701, issued to K. Knirsch et al. on Aug. 18, 1987, discloses a heat sensitive inked element using a blend of thermoplastic resins and waxes.
U.S. Pat. No. 4,777,079, issued to M. Nagamoto et al. on Oct. 11, 1988, discloses an image transfer type thermosensitive recording medium using thermosoftening resins and a coloring agent.
U.S. Pat. No. 4,865,901, issued to Ohno et al. on Sep. 12, 1989, discloses a thermal transfer printing ribbon with an ink layer comprising a blend of ethylene-vinyl acetate copolymer and a viscous resin as a binder with correction/erasability capabilities.
U.S. Pat. No. 4,983,446, issued to Taniguchi et al. on Jan. 8, 1991, describes a thermal image transfer recording medium which comprises as a main component, a saturated linear polyester resin.
Ultraviolet radiation curable inks are known and most comprise a reactive oligomer, a reactive monomer, a photoinitiator, a pigment and optional additives. UV curable inks are commonly used in printing methods other than thermal transfer printing, such as screen printing, and lithography techniques for printed circuit boards, examples being described in U.S. Pat. Nos. 5,200,438; 5,391,685; 5,270,368; 4,680,368 and 5,500,040. UV curable inks said to be suitable for ink jet printing are described in U.S. Pat. Nos. 4,258,367 and 5,641,346. Conventional UV curable inks typically do not have the transfer properties necessary for use in conventional thermal transfer printing processes with conventional thermal transfer printers after cure. They are typically formulated for use in printing methods wherein curing provides a permanent image.
To be suitable for thermal transfer printing, there are many requirements placed on conventional general purpose thermal transfer media and the coating formulations which produce them. For example, the properties of the thermal transfer layer must permit transfer from a carrier to a receiving substrate and provide a stable, preferably permanent image. The release properties and adhesive properties needed to meet these requirements are in conflict and typically require a mixture of components to address both needs. As the use of thermal transfer printing grows into new applications, the requirements for the ribbons become broader and more strict. For example, when printing on rough stock, very high adhesive properties are required from the transferred image since only a portion of the image may contact high spots on the substrate surface. Conventional general purpose ribbons with a single layer often cannot meet these requirements. Two separate layers are typically needed to provide the required release properties and adhesive properties. Applying two separate layers to form the thermal transfer medium significantly adds to the cost of manufacture. It is desirable to prepare thermal transfer media which will form images on rough stock and does not require the application of two separate layers for their production.
Conventional general purpose ribbons with a single layer also cannot meet the requirements of high speed printers known in the art as "near edge", "true edge" and "Fethr.TM." printers, referred to herein collectively as "high speed printers", due to the rapid separation of the ribbon from the substrate once the print head heating elements have been fired. Since the ribbon and receiving substrate are separated almost instantaneously after the thin film resistors are fired, there is very little time for waxes and/or resins to melt/soften and flow onto the surface of the receiving substrate before the ribbon is separated from the receiving substrate. With conventional single ink layer ribbons, the ink layer is usually split and the transfer incomplete, resulting in light printed images. Two separate layers are typically needed to provide the required release properties and adhesive properties. Applying two separate layers to form the thermal transfer medium significantly adds to the cost of manufacture. It is desirable to prepare thermal transfer media which will form images with a high speed printer and do not require the application of two separate layers for their production.
In another aspect of thermal transfer printing, extensive work has been done to develop water-rich systems to replace organic solvent-based systems. Water-based and water-rich coating formulations improve safety, reduce costs, and simplify compliance with environmental regulations and restrictions. For example, U.S. Pat. No. 4,923,749 issued to Talvalkar, discloses a thermal transfer ribbon which comprises a thermal sensitive layer and protective layer, both of which are water-based. It is desirable to prepare thermal transfer layers from a coating formulation which does not require any solvent, whether aqueous or organic.